1. Field of the Invention
This invention relates to a displacement detection apparatus for detecting the relative position of a moving apparatus, which may be a machine tool, an inspection apparatus or a semiconductor manufacturing system.
2. Description of the Related Art
Displacement detection apparatus for detecting the relative position of a moving apparatus, which may be a machine tool or a semiconductor manufacturing system, are known. Such a displacement detection apparatus comprises a scale on which a periodic signal having a constant wavelength is recorded in a predetermined direction with a fluctuating signal level, a head section for detecting the periodic signal recorded on the scale and a processing section for processing the periodic signal detected by the head section and outputting positional information. The scale and the head section are fitted to respective members of a movable section and a reference section that are adapted to move relative to each other. With the displacement detection apparatus, the head section detects the periodic signal whose signal level fluctuates on the scale in response to the relative movement of the two members and supplies the detected periodic signal to the processing section. The processing section by turn outputs positional information that indicates the relative positions of the relatively moving two members according to the periodic signal detected by the head section.
Known displacement detection apparatus include those having a single scale and a single head section. FIGS. 1A through 1D of the accompanying drawings schematically illustrate a known displacement detection apparatus mounted on a machine tool that is driven to move linearly. On the machine tool, the single head 100 of the displacement detection apparatus is arranged in a central part of a bed 101, which is a stationary member, and vis-à-vis a scale 102, which will be described in greater detail hereinafter. The scale 102 is arranged longitudinally under a table 103, which is a movable member adapted to move linearly either in the direction of arrow A from left L to right R or in the direction of arrow B from right R to left L, and vis-à-vis the head 100. The scale 102 and the head 100 may be a magnetic scale and a magnetic modulation type magnetic head or an optical scale and an optical head.
FIG. 1A illustrates a situation where the table 103 is moved relative to the head 101 in the direction of arrow B and the scale 102 is located at its left marginal position. The head 100 is located vis-à-vis the right side end of the scale 102. FIG. 1B illustrates a situation where the table 103 is moving in the direction of arrow A and located vis-à-vis a central part of the bed 101 and hence the scale 102 is also located vis-à-vis a central part of the bed 102. Accordingly, the head 100 is located vis-à-vis the middle point of the scale 102. FIG. 1C illustrates a situation where the table 103 is moved further relative to the head 101 in the direction of arrow A and the scale 102 is located at its right marginal position. The head 100 is located vis-à-vis the left side end of the scale 102. FIG. 1D illustrates a situation where the scale 102 is located at its left marginal position relative to the head 100 as overlapped on a situation where the scale 102 is located at its right marginal position relative to the head 100. The bed 101 has a length (bed length: BL) twice as long as the measurable length ML of the scale 102.
Japanese Patent Application Laid-open Publication No. 10-239105 (Patent Document 1) discloses a displacement detection apparatus that is a magnetic scale apparatus formed by lining a plurality of magnetic scales and adapted to take out phase modulation signals by means of a pair of integrally incorporated magnetic transducers.
FIG. 2 schematically illustrates a magnetic scale apparatus as disclosed in the Patent Document 1. Referring to FIG. 2, a magnetic scale substrate 110 carries thereon a magnetic scale 111 realized by using a magnetic medium formed on a strip-shaped plate and a signal having a constant wavelength λ is recorded on the magnetic scale 111. A magnetic transducer 112 formed by using a pair of magnetic modulation type magnetic heads is fitted to a bed and arranged vis-à-vis the magnetic scale 111.
Referring again to FIG. 2, another magnetic scale substrate 113 has a configuration similar to that of the magnetic scale substrate 110. A magnetic transducer 114 formed by using a pair of magnetic modulation type magnetic heads is fitted to a bed and arranged vis-à-vis the magnetic scale substrate 113. The two magnetic scale substrates 110 and 113 are linked to each other so as to be used as a long scale. The two magnetic transducers 112 and 114 are also integrated with each other to form a head section 115.
The pairs of magnetic heads of the two magnetic transducers 112 and 114 are supplied with an excitation signal from an excitation circuit 116. The pairs of magnetic heads of the two magnetic transducers 112 and 114 are connected respectively to a first detection circuit 117 and to a second detection circuit 118. Each of the first detection circuit 117 and the second detection circuit 118 detects the phase modulation signal taken out in response to the relative positional change of the corresponding magnetic transducer relative to the corresponding magnetic scale and carries out an interpolation process on the signal before outputting it. Output circuit 119 selectively outputs the output of one of the detection circuits to provide positional information.
FIG. 3 schematically illustrates the positional relationship of the scale substrates 110 and 113 and that of the magnetic transducers 114 and 112 of the head section 115 of the above described magnetic scale apparatus. Assume here that the head section 115 is moving rightward relative to the scale substrate 110. While both of the magnetic transducers 114 and 112 provide outputs in the illustrated position, the output of the magnetic transducer 112 is selected for use.
As the magnetic transducer 112 comes into the link section 120 of the two scale substrates 110 and 113, the output of the magnetic transducer 112 is reduced so that the output circuit 118 switches from the use of the output of the magnetic transducer 112 to the use of the output of the magnetic transducer 114.
As the two transducers move further and the magnetic transducer 114 comes into the link section 120, the output of the magnetic transducer 114 is reduced this time but the magnetic transducer 112 has already moved out of the link section 120 and recovered the output so that the output circuit 118 switches from the use of the output of the magnetic transducer 114 to the use of the output of the magnetic transducer 112.
In this way, the above described magnetic scale apparatus avoids signal degradation in the link section 120 by selecting the output of the transducer that is providing a good output.